The operation of inkjet printing devices relies on stable surface properties of particular components, including nozzle plate surfaces, nozzle bore surfaces, and surfaces of drop catching mechanisms, such as gutters or drop catchers. For example, Coleman et al. in U.S. Pat. No. 6,127,198 discuss the need to have hydrophilic surfaces internal to the fluid injector of an ink jet device and hydrophobic properties on exterior surfaces such as the nozzle front face. Bowling in U.S. Pat. No. 6,926,394 describes the need for a hydrophobic surface on a drop catcher for continuous ink jet printers.
The surface properties of a component are affected by its surface chemical composition and degree of contamination from a variety of sources, such as hydrocarbon compounds in the room air, debris such as skin flakes and dust particles, and deposited particulate from inks. Consequently, cleaning and maintenance of inkjet print device components is critical to consistent printing performance.
One common technique to clean surfaces for inkjet printing devices includes washing in a cleaning solution, see, for example, Sharma et al, U.S. Pat. No. 6,193,352; Fassler et al., U.S. Pat. No. 6,726,304, and Andersen, U.S. Pat. No. 5,790,146. However, washing inkjet device components in cleaning solutions is not a practicable maintenance approach, as it requires providing a bath of cleaning solution and generally requires removal of the device from the printer. Hence, it is preferable to apply surface coatings to device components and to clean the device components by techniques that can be implemented in-situ.
Another common technique to prepare surfaces for inkjet printing devices includes applying hydrophobic or lyophobic coatings like those described in Coleman et al., U.S. Pat. No. 6,127,198 (diamond-like carbon with fluorinated hydrocarbon); Yang et al. in U.S. Pat. No. 6,325,490 (self assembled monolayers of hydrophobic alkyl thiols); Drews, U.S. Pat. No. 5,136,310 (alkyl polysiloxanes and variants thereof); Narang et al., U.S. Pat. No. 5,218,381 (silicone doped epoxy resins); and Skinner et al., U.S. Pat. No. 6,488,357 (gold, coated with an organic sulfur compound). However, this approach has limitations. For example, coatings tend to foul with device usage.
Another common technique for surface cleaning includes wiping surfaces with “blades” of rubber or some other suitably soft material, see, for example, Dietl et al., U.S. Pat. No. 6,517,187; and Mori et al. US Patent Application Publication No. 2005/0185016. However, this approach has limitations. For example, wiping can eventually degrade the non-wetting character of the device surface.
Given the limitations of current approaches to maintaining critical surface properties of inkjet printing device components, it would be advantageous to clean and prepare surfaces on components of fully assembled printing devices without having to remove them so that desirable surface conditions could be restored or maintained periodically or as needed. It would also be advantageous to use processes with reduced materials and energy consumption.
Plasma processes for coating and cleaning in general make more efficient use of materials than liquid-based processes. Furthermore, a wide variety of materials can be prepared and deposited using plasmas. For example, polymer materials can be formed by plasma polymerization by feeding monomer material into a plasma environment, as described in Plasma Polymerization, H. Yasuda, Academic 1985; by Kuhman et al. in U.S. Pat. No. 6,444,275 (depositing fluoropolymer films on thermal ink jet devices); and by DeFosse et al. in U.S. Pat. No. 6,666,449 (depositing fluoropolymer films on star wheel surfaces).
Kuhman et al. in U.S. Pat. No. 6,243,112 also describe the use of plasma processes to deposit diamond-like carbon, and further using plasma processing in fluorine bearing gases to fluorinate the diamond-like carbon film. Semiconductor (e.g., Si) oxides or nitrides and metal (e.g., Ta) oxides or nitrides can be deposited by feeding semiconductor or metal bearing precursor vapor and respective oxygen or nitrogen bearing gas into a plasma environment, as discussed by Martinu and Poitras (J. Vac. Sci. Technol. A 18(6), 2619-2645 (2000)); Kaganowicz et al. in U.S. Pat. No. 4,717,631 (describing the use of plasma enhanced chemical vapor deposition (PECVD) to form silicon oxynitride passivation layers from a mixture of SiH4, NH3, and N2O precursors); Hess in U.S. Pat. No. 4,719,477 (describing the use of PECVD to deposit silicon nitride on tungsten conductive traces in fabrication of a thermal ink jet printhead); and Shaw et al. in U.S. Pat. No. 5,610,335 (describing the use of PECVD oxide to passivate trench sidewalls in fabrication of a micromechanical accelerometer).
Plasmas are also well known for etching and cleaning applications. Oxygen bearing plasmas in particular are well known for removal of organic and hydrocarbon residue, see, for example, Fletcher et al, U.S. Pat. No. 4,088,926, Williamson et al., U.S. Pat. No. 5,514,936), and for removal (commonly referred to as ashing) of residual photoresist materials in semiconductor processing, see, for example, Christensen et al., U.S. Pat. No. 3,705,055, Mitzel, U.S. Pat. No. 3,875,068, Bersin et al., U.S. Pat. No. 3,879,597, and Muller et al., U.S. Pat. No. 4,740,410.
In common plasma processing as described above, the cleaning, etching, or deposition process is carried out at reduced pressure (typically below 2 mBar, or 200 Pa, or roughly 1.5 Torr), thus requiring the treatment process to be carried out in a vacuum chamber. Because of the controlled environment that the vacuum enclosure affords, a wide variety of etching, cleaning, surface chemical modification, and deposition processes are readily practicable in these low-pressure plasma processes.
Atmospheric pressure plasmas are also known. In contrast to the low-pressure plasma processes, plasmas run in ambient air are generally limited to cleaning and surface chemical modification processes based on activated oxygen species. Typical atmospheric pressure plasmas used in industrial applications are corona discharges and dielectric barrier discharges. The dielectric barrier discharge, in particular, is well known in ozone generation for water purification and for polymer surface modification applications in coating, lamination, and metallization processes. In contrast to low-pressure plasmas, which operate at values of Pd (the product of pressure P and electrode gap d) below the minimum on the Paschen curve (i.e., the break down voltage Vas a function of Pd), these high-pressure plasmas operate at Pd values above the minimum in the curve and typically operate an order of magnitude higher in applied voltage. While the corona discharge has diffuse glow-like characteristics, it typically can support low power densities. The dielectric barrier discharge, typically driven at low radio frequency (i.e., approximately 10 kHz to 100 kHz) to mid radio frequency (i.e., approximately 100 kHz to 1 MHz) can support higher power densities, and electrical breakdown proceeds by avalanche effects and streamer formation. Local charging of the dielectric barrier sets up an opposing electric field that shuts down the streamers and prevents formation of arcs (high-current, low-voltage discharges where the gas is heated sufficiently to produce significant ionization). By alternating the high voltage applied to the discharge gap, streamers are formed in opposite directions each half cycle. The dielectric barrier discharge has proven useful in the printing industry as a means of modifying substrates surfaces to accept inks. The high voltage operation (10 kV or greater) and the filamentary nature of this discharge present serious limitations for extending this technology to other applications.
While atmospheric pressure plasmas, such as DBDs are often applied in surface modification of polymers and in treatment of gases for pollution abatement, atmospheric pressure plasmas have also been developed for plasma deposition processes. Examples include the DBD-based process described by Slootman et al. in U.S. Pat. No. 5,576,076 for coating SiOx in roll-to-roll format; APGD to deposit thin fluorocarbon layers on organic light emitting diode devices as described by Sieber et al., in U.S. Pat. No. 7,041,608; and hybrid hollow cathode microwave discharges to deposit diamond-like carbon described by Bardos and Barankova, in “Characterization of Hybrid Atmospheric Plasma in Air and Nitrogen”, Vacuum Technology & Coating 7(12) 44-47 (2006).
In large-area plasma modification processes, the high operating voltages and spatial non-uniformity of the dielectric barrier discharges (DBDs) have often proven undesirable. Efforts to achieve the uniform glow-like character of low-pressure discharges at atmospheric pressure (atmospheric pressure glow discharge or APGD) have used a variety of techniques, including adding helium and other atomic gases to dielectric barrier discharges and/or carefully selecting driving frequency and impedance matching conditions under which a dielectric barrier discharge is run, see, for example, Uchiyama et al, U.S. Pat. No. 5,124,173; Roth et al., U.S. Pat. No. 5,414,324; and Romach et al., U.S. Pat. No. 5,714,308. Other approaches not requiring a dielectric barrier include using helium and radiofrequency power (e.g., 13.56 MHz) in combination with appropriate electrode configuration, see, for example, Selwyn, U.S. Pat. No. 5,961,772 (describing an atmospheric pressure plasma jet), and scaling a plasma source to dimensions at which Pd values nearer the Paschen minimum can be achieved at higher pressures than typical low-pressure discharges, see, for example, Eden et al. U.S. Pat. No. 6,695,664 and Cooper et al., US Patent Application Publication No. 2004/0144733 (describing microhollow cathode discharges).
In typical plasma cleaning and plasma treatment processes, the article to be treated or cleaned is either placed in a treatment chamber wherein plasma is generated (i.e. a process with stationary substrates), or it is conveyed through a plasma zone (i.e., a process with translating substrates). An example of the former mode of process is plasma ashing of photoresist in semiconductor manufacturing (see previously cited references). In these applications, the electrode system is generally independent of the article to be treated, and the surface of the article is generally at floating potential (i.e., the potential that an electrically insulated object naturally acquires when presented to the plasma, such that the object draws no net electrical current; generally this potential is approximately 10-20 volts below the plasma potential, the difference depending on the electron temperature in the plasma, see, for example, Principles of Plasma Discharges and Materials Processing, by M. A. Lieberman and A. J. Lichtenberg, Wiley, New York (1994). An example of the latter mode, wherein the article to be treated is conveyed through a plasma zone, is plasma treatment of polymer webs, see, for example, Grace et al., U.S. Pat. No. 5,425,980; Tamaki et al., U.S. Pat. No. 4,472,467; and Denes et al., U.S. Pat. No. 6,082,292.
In some web treatment techniques, the web is electrically floating whereas in other techniques, the web is placed in the cathode sheath, see, for example, Grace et al., U.S. Pat. No. 6,603,121; and Grace et al., U.S. Pat. No. 6,399,159, and experiences energetic bombardment from ions accelerated through the high-voltage sheath (as is typical in plasma etching processes used in fabrication of microelectronic circuits on silicon wafers). In these approaches, the entire substrate surface presented to the plasma is treated. Furthermore, neither of these approaches is compatible with treating inkjet printing device components without removing them from the inkjet printing system.
Regardless of pressure range of operation, typical plasma processing techniques employ macroscopic plasmas, and the process powers and areas tend to be high. For example, typical power supplies for etching semiconductor wafers are capable of delivering 1-5 kW and wafer areas are typically in the range 180 cm2 to 700 cm2. Power supplies for plasma web treatment devices generally are capable of delivering 1-10 kW for web widths of 1-2 m and treatment zones of order 0.3 m long. Consequently, adapting such large-scale approaches to processing only a small fraction of a device surface area would make inefficient use of energy and would possibly limit the process speed for lack of ability to provide required local energy densities, which would need to be applied over the large volumes or areas involved in such large-scale approaches. Additionally, plasma sensitive components in the device can be damaged by exposure of the device to large-scale plasmas.
Micro-scale plasmas (i.e., a plasma characterized by having sub-millimeter extent in at least one dimension) provide localized plasma processing and, as mentioned above, higher operating pressures by virtue of Pd scaling. An example of localized plasma processing using micro-scale plasmas is the use of patterned plasma electrodes to produce micro-scale plasma regions over a substrate to add material or remove material in a desired pattern, as described by Gianchandani et al. in U.S. Pat. No. 6,827,870. Etch process results are disclosed for applied power densities in the range 1-7 W/cm2 and gas pressures in the range 2-20 Torr. While these pressures are significantly higher than traditional low-pressure plasma processes (i.e., <1 Torr), they are considerably lower than atmospheric pressure (760 Torr) and, therefore, Gianchandani does not teach or disclose the design of the micro-scale discharge source to operate at near atmospheric pressures.
The micro-hollow-cathode source of Cooper et al. is aimed at providing intense ultraviolet light for water purification and is shown to operate at higher pressures (200-760 Torr) than disclosed by Gianchandani. The object of the more recently disclosed micro-hollow-cathode source of Mohamed et al., US Patent Application Publication No. US 2006/0028145 is to produce a micro plasma jet at atmospheric pressure. In the former case, the ability to produce the requisite ultraviolet emission depends on the choice of discharge gas and operating conditions of the device. In the latter case, the microhollow cathode device also serves as a gas nozzle, and the jet characteristics depend on nozzle design and flow conditions as well as the plasma conditions.
Other examples of atmospheric pressure micro-scale plasma sources include the plasma needle described by Stoffels et al. (Superficial treatment of mammalian cells using plasma needle; Stoffels, E.; Kieft, I. E.; Sladek, R. E. J. Journal of Physics D: Applied Physics (2003), 36(23), 2908-2913), the narrow plasma jet disclosed by Coulombe et al., US Patent Application Publication No. 2007/0029500; the microcavity array of Eden et al., US Patent Application Publication No. S 2003/0132693; the multilayer ceramic microdischarge device described by Vojak et al., US Patent Application Publication 2002/0113553; and the low-power plasma generator of Hopwood et al., US Patent Application Publication No. 2004/0164682. The plasma needle of Stoffels et al. is aimed at surface modification of living cells in mammalian tissue. The narrow plasma jet of Coulombe et al. is also directed toward biological applications, such as skin treatment, etching of cancer cells and deposition of organic films. The microcavity array of Eden et al. is aimed at light emitting devices, and the multilayer ceramic microdischarge device of Vojak et al is directed toward light emitting devices or microdischarge devices integrated with multilayer ceramic integrated circuits. The low power plasma generator of Hopwood et al., which employs a high-Q resonant ring with a discharge gap, is directed towards portable devices and applications such as bio-sterilization, small-scale processing, and microchemical analysis systems. In addition to the glow-like character of these discharges, they generally operate at or near atmospheric pressure, and they are spatially localized. Hence, plasma processing of selected localized areas at atmospheric pressure, with operating characteristics similar to low pressure plasmas is possible.
The micro-scale atmospheric pressure plasma sources mentioned above might produce useful localized plasma processing for cleaning or treatment of ink jet printing device components. In none of these cases is there mention of applying plasma treatment selectively to localized areas of a printer component or device, such as an ink jet print head, that contains sensitive electronics, such as CMOS logic and drivers, nor is their concern for rapid processing times that would require generation of significant localized fluxes of reactive species in specific regions of a component in order to process the component in with reasonable process time and minimal damage thereto. Furthermore, none of these cases teaches integration of the micro-scale discharge electrode system directly into a device designed for printing, wherein components of the printing device serve as part of the electrode system for generation of the plasma, nor do they teach the use of micro-scale discharges to clean, prepare, or otherwise maintain the surface properties of inkjet printing components.
While one of ordinary skill in the art of printing might be familiar with dielectric barrier discharges or variants thereof for surface treatment of printing substrates because printing processes run at atmospheric pressure, most plasma processes that run under vacuum conditions would be considered prohibitive from the standpoint of workflow and capital cost. The ability to run a plasma process at atmospheric pressure with characteristics similar to those of vacuum plasma processes and with the potential to introduce specific plasma chemistries tailored for cleaning, etching, or deposition is highly desirable and is not known in the printing art. It is further desirable to have the ability to carry out such processes effectively, using geometries compatible with inkjet printer components, without mechanical or electrical damage to critical components of the printing system. The integration of plasma technologies into the printing system for applications other than printing or substrate modification is highly desirable.
Thus, there is a need for a plasma treatment process integrated with an inkjet printing system and operable without causing damage to printing device components.